(1) Field of the Invention
This invention relates to the fabrication methods used for semiconductor devices, and more specifically to an optimized tungsten stud process, used to interconnect active device regions to overlying metallizations.
(2) Description of Prior Art
The semiconductor industry is continually striving to increase the performance of semiconductor devices, while still attempting to reduce the cost of these higher performing devices. The trend to micro-miniaturazation, or the ability to produce semiconductor devices with sub-micron features, has allowed the increased performance and cost reduction objectives to be realized. The ability to fabricate silicon chips with sub-micron features has yielded higher performing devices, via reductions in performance degrading resistances and capacitances. In addition sub-micron features, resulting in smaller silicon chips, has allowed more silicon chips to be obtained from a specific size starting silicon wafer. Since the cost of processing the silicon wafer remains unchanged, the cost of a specific chip is reduced. Many semiconductor fabrication disciplines, such as photolithography and anisotropic, dry etching, have been major contributors to the realization of micro-miniaturazation. For example, more sophisticated exposure cameras, as well as more sensitive photoresist materials, have allowed sub-micron features to be routinely achieved in photoresist masking layers. In addition advances in selective, anisotropic, reactive ion etching procedures, have in turn allowed the sub-micron images in overlying photoresist masking layers, to be successfully transferred to underlying materials, used for fabrication of semiconductor devices.
However with the advent of micro-miniaturazation, specific silicon device structures and features become more vulnerable to yield and reliability problems. For example the higher aspect ratio of contact holes, and via holes, fabricated with sub-micron features, present problems when attempting to fill these deep, narrow diameter holes, using conventional metal deposition processes. In addition smaller contact and via holes place more stringent demands on the current carrying capabilities of the metal used to fill the sub-micron contact holes. Therefore a trend away from the use of aluminum based metallizations, to fill sub-micron contact and via holes, has occurred. The inability of sputtered aluminum to adequately fill the high aspect ratio contact holes, and the inability of aluminum to reliably sustain the higher, contact hole current densities, has led to the use of tungsten as a contact hole fill material. Tungsten offers greater current carrying capabilities then aluminum, and thus is less prone to electromigration reliability failures. In addition tungsten can be deposited using low pressure chemical vapor deposition, (LPCVD), processing, offering more conformal, contact hole filling, then aluminum counterparts, thus offering more yield potential. However even tungsten depositions, using LPCVD processes, can result in undesirable fill features. A basic characteristic of an LPCVD contact hole fill is by deposition of the metal on the sides of the deep, narrow contact hole. Sometimes at the completion of the metal fill, a seam or void is present at the point where the depositing metal, on the sides of the contact hole, meet. This void or seam has been observed for high aspect ratio contact holes, filled with tungsten. Subsequent exposure of the tungsten void or seam, to dry etching processes, used for patterning purposes, aggravates or increases the size of the void. This defect is transferred to subsequent metallizations, overlying the tungsten filled contact hole, resulting in possible electromigration, reliability problems.
This invention will describe a process for successfully filling high aspect ratio contact holes with tungsten, via the use a selective redeposition of tungsten used to fill voids or seams created with the initial LPCVD tungsten fill. Cheffings, et al, in U.S. Pat. No. 5,387,550, describe a process for filling voids or seams, in tungsten filled contact holes, with silicon. However that process requires the complexity and cost of using an etchback or planarazation process to remove silicon from unwanted regions. This invention will describe a selective tungsten deposition process, requiring no additional etchback or planarazation processing.